DC Power System Conductor Ampacity

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gonzoleroy

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I maintain a lab that utilizes a -48 VDC system for telecom equipment. The system was installed ~12 years ago. Nearly all of my experience is with AC power, so I am fairly unfamiliar with DC requirements as governed by the Code.

While addressing a request to add additional drops to the lab it was brought to my attention that all of the 100 or so DC circuits in this lab are supplied with standard #6 Cu, THHN via VertiRack--and protected by 100-amp circuit breakers. As a raceway type of guy, this level of current on #6 immediately bothered me, so I dove into the NEC to gain understanding.

It seems that whomever designed this lab referred to Table 310.17 for allowable ampacity--which specifies "Conductors...In Free Air". Have a look at distribution in these Vertiracks:

Lab Wiring.JPG

There is clearly no spacing maintained, the conductors are simply bundled onto the Vertirack. How can Table 310.17 be used in this instance?

Article 392.11(B) mentions ampacity for sizes 1/0 and above, and also describes the required spacing. Am I missing some provision for wire sizes smaller than 1/0 AWG?

Any help you guys can provide is appreciated.
 
gonzoleroy said:
I maintain a lab that utilizes a -48 VDC system for telecom equipment. The system was installed ~12 years ago. Nearly all of my experience is with AC power, so I am fairly unfamiliar with DC requirements as governed by the Code.
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Donrescuecaptain has posted before that the NEC does not differentiate between a/c and d/c as far as what is required. I agree with that.
 
qcroanoke said:
Donrescuecaptain has posted before that the NEC does not differentiate between a/c and d/c as far as what is required. I agree with that.
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Fair enough, but my issue deals with the "Free Air" aspect of T310.17. I am concerned that 392.11(B) was incorrectly interpreted and T310.17 may not apply in this instance. Thoughts?
 
qcroanoke said:
Donrescuecaptain has posted before that the NEC does not differentiate between a/c and d/c as far as what is required. I agree with that.
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As a practical matter, in large conductor sizes the ampaciry for DC will be slightly higher because of the AC skin effect. But the NEC tables do not take that into account.
 
ron said:
I don't recall the tray fill limits in the NEC (referenced below), but the derating of those condcutors will be severe.

Here is a good resource http://www.cooperindustries.com/con.../cable_tray_systems/all_products/CTManual.pdf
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Just from memory, the allowable fill for small conductors is something like 38% for up to 3" depth of cables in a ventilated tray. This type of rack/tray seems to be extremely well ventilated. But I am willing to bet a non-trivial amount of money that the fill limits have been exceeded. If you exceed the fill limits, the derating factors are not applicable, since you are not allowed to use that configuration at all.

Looking at the multiple empty parallel loops of the horizontal run, I would also guess that the engineer who laid out the system intended that number of cables to be distributed over four separate horizontal runs instead of being bundled into one. Whoever installed the cables ignored that, or else it was not clear in the drawings.

All that is saving this installation from disaster, although not making it code compliant, is likely that the conductors are never all being loaded to near their ampacity at the same time.
There is also the factor that the installation is probably not governed by NEC in the first place, but there will be corporate standards and practices to fill that role instead. Do you have access to those?
 
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ron said:
Do you think it doesn't need to comply to the NEC because it is <50V?
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Nope. Because, as qcroanoke pointed out, it may be covered by this exemption in 90.2(B)(4):
(4) Installations of communications equipment under the exclusive control of communications utilities located outdoors or in building spaces used exclusively for such installations
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Note: not necessarily the whole building.

If the company is not itself a communications utility, then that can be ignored. :)
 
GoldDigger said:
Just from memory, the allowable fill for small conductors is something like 38% for up to 3" depth of cables in a ventilated tray. This type of rack/tray seems to be extremely well ventilated. But I am willing to bet a non-trivial amount of money that the fill limits have been exceeded. If you exceed the fill limits, the derating factors are not applicable, since you are not allowed to use that configuration at all.

Looking at the multiple empty parallel loops of the horizontal run, I would also guess that the engineer who laid out the system intended that number of cables to be distributed over four separate horizontal runs instead of being bundled into one. Whoever installed the cables ignored that, or else it was not clear in the drawings.

All that is saving this installation from disaster, although not making it code compliant, is likely that the conductors are never all being loaded to near their ampacity at the same time.
There is also the factor that the installation is probably not governed by NEC in the first place, but there will be corporate standards and practices to fill that role instead. Do you have access to those?
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I think you nailed it with installation being done incorrectly (one large bundle instead of multiple). And yes, the individual circuits are rarely loaded near full capacity--and only for brief periods in those instances.

As for corporate standards, there is no guidance specific to this system. Whoever was PM for this buildout likely relied upon the designing engineer for NEC compliance.

Thanks very much for the insight. This will be a great help as I proceed with adding additional power to this lab.
 
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